(a) Field of the Invention
This invention relates to lubricants for powder metallurgy and to the manufacture and use of lubricants.
More particularly the lubricant comprises an admixture of particulate lubricants comprising discrete particles of a fatty monoamide, especially oleamide as one of the components.
(b) Description of Prior Art
Powdered metals, for example, powdered iron, are used to make small, fairly intricate parts, for example, gears. The fabrication of such metallic parts by powdered metal technology involves the following steps:
(a) the powdered metal is blended with a lubricant and other additives to form a mixture,
(b) the mixture is poured into a mold,
(c) the mixture is compacted in the mold to form a part using high pressure, usually of the order of 30 tons per square inch,
(d) after compaction the part is ejected from the mold,
(e) the ejected part is subjected to a high temperature to decompose and remove the lubricant,
(f) the part is heated to a higher temperature to cause all of the particles of metal in the part to sinter together and
(g) the part is cooled, after which it is ready for use.
Commonly used lubricants include lithium stearate, lithium 12-hydroxystearate, ethylenebisstearamide, and stearic acid.
The lubricant is added to the powdered metal for several reasons; in particular the lubricant increases the bulk density of the uncompacted powdered metal. This means that the molds can be shallower, for a given thickness of the final part. The bulk density is generally referred to as the apparent density and is determined according to the Metal Powder Industries Federation Standard No. 04, Determination of Apparent Density of Free-Flowing Metal Powders Using the Hall Apparatus.
Some lubricants increase the rate of addition of the metal powder to the mold, when admixed with the powder. A standard laboratory test for this is the time taken for 50.0 grams of metal powder with admixed lubricant to flow through a standard cup. This property is commonly referred to as the flow rate of the mixture and is determined as described by the Metal Powder Industries Federation Standard No. 03, Determination of Flow Rate of Free-Flowing Metal Powders Using the Hall Apparatus.
The lubricant allows the compacting pressure to be reduced to attain a specified density before sintering. This is very important because it means that for a given pressure a larger part can be made. Because of the very large pressures required to compact powdered metal, only relatively small parts are made. The density of the compacted (pre-sintered) part is called the green density.
The strength of the compacted (pre-sintered) part is called the green strength of the part. It can be determined as described by the Metal Powder Industries Federation Standard No. 15, Determination of Green Strength of Compacted Metal Powder Specimens.
The ejection force to remove the compacted part from the mold is much lower when a lubricant is present and this lower force results in less mold wear.
Unfortunately, the lubricant also has a few adverse effects; some lubricants reduce the flow rate of the powdered metal and therefore the rate at which a mold can be filled; the lubricant may reduce the strength of the compacted (pre-sintered) part, referred to as the green strength; further, the lubricant can cause an unattractive surface finish on the sintered part. Zinc stearate is commonly used as a lubricant and slowly deposits a thin coating of zinc and zinc oxide on the walls of the furnace used to burn off the lubricant or on the walls of the sintering furnace.
This last disadvantage is often serious, and because of it a wax is sometimes used instead of zinc stearate. The most commonly used wax is ethylenebisstearamide; however, it is not as good a lubricant as zinc stearate, especially with regard to compressibility, i.e., it gives a lower green density for a given compacting pressure. It can only provide the same compressibility as zinc stearate if it is ground to a very fine powder using a special grinding mill which is expensive and consumes a great deal of energy.
A further disadvantage of zinc stearate is that some of the zinc oxide that results from its decomposition in the sintering furnace is also vented into the atmosphere. The amount of zinc oxide emissions is monitored by regulatory authorities. Installation of pollution abatement equipment is costly and thus minimizing the production of evolved zinc oxide at the source is a more desirable approach. For these reasons, the powder metallurgy industry is migrating away from zinc stearate-containing lubricants wherever possible. Hence, zinc stearate-containing compositions are not considered in this invention.
U.S. Pat. Nos. 5,368,630 and 5,429,792 describe lubricated metal powder compositions which contain an organic binder. The compositions are designed for high temperature use above 100° C. The organic binder is an essential component to achieve dust-free, segregation-free metal powder compositions. The binding agent is introduced in a solvent which is subsequently removed from the powder metal composition. The U.S. Patents teach that not all conventional powder metallurgy lubricants may be employed where compaction is carried out at the high temperature.